Selective placement of carbon nanotubes through functionalization

ABSTRACT

The present invention provides a method for selectively placing carbon nanotubes on a substrate surface by using functionalized carbon nanotubes having an organic compound that is covalently bonded to such carbon nanotubes. The organic compound comprises at least two functional groups, the first of which is capable of forming covalent bonds with carbon nanotubes, and the second of which is capable of selectively bonding metal oxides. Such functionalized carbon nanotubes are contacted with a substrate surface that has at least one portion containing a metal oxide. The second functional group of the organic compound selectively bonds to the metal oxide, so as to selectively place the functionalized carbon nanotubes on the at least one portion of the substrate surface that comprises the metal oxide.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/324,441, filed Jan. 3, 2006.

FIELD OF THE INVENTION

The present invention generally relates to the selective placement ofcarbon nanotubes on a particular surface. More particularly, the presentinvention provides a method that uses functionalized carbon nanotubescapable of selectively bonding to surfaces that comprise a metal oxide.The present invention also relates to compositions that contain suchfunctionalized carbon nanotubes, as well as materials that can be usedfor forming such functionalized carbon nanotubes.

BACKGROUND

In the field of molecular electronics, few materials show as muchpromise as carbon nanotubes that comprise hollow cylinders of graphitethat have a diameter of a few Angstroms. Carbon nanotubes have excellentelectrical properties, which make them attractive for applications innanotechnology.

Semiconducting carbon nanotubes, in particular, have received attention,due to their promising performance in electronic devices, such as diodesand transistors. For example, semiconducting carbon nanotubes can beused as channels in field effect transistors (FETs). Therefore,semiconducting carbon nanotubes are considered to be one of the mostpromising candidate materials for making nano-sized semiconductorcircuits.

The most common prior art method of fabricating carbon nanotube FETsstarts with depositing a carbon nanotube on a thin oxide film from aliquid suspension. Source and drain contacts are then formedlithographically on the nanotube to form a FET device.

An exemplary prior art carbon nanotube FET device 10 is illustrativelyshown in FIG. 1. Specifically, the bulk Si substrate 12 functions as aback gate. The thin oxide film 14, onto which the carbon nanotube 18 isdeposited, functions as the gate dielectric. Source and drain contacts16 a and 16 b are formed over the gate dielectric 14 at two terminalends of the carbon nanotube 18. In this manner, the carbon nanotube 18bridges between the source and drain contacts 16 a and 16 b, so it canfunction as the channel in the FET device 10.

The deposition of carbon nanotubes on an oxide surface, followed bylithographic patterning of the source and drain contacts, has beensuccessfully used in the prior art for the construction of single carbonnanotube FETs. However, fabrication of integrated circuits fromnanotubes requires the precise placement and alignment of large numbersof carbon nanotubes on a surface (e.g., spanning the source and draincontacts). E. Valentin, et al., “High-density selective placementmethods for carbon nanotubes”, Microelectronic Engineering, 61-62(2002), pp. 491-496 disclose a method in which the adhesion of carbonnanotubes onto a SiO₂ surface is improved usingaminopropyltriethoxysilane (APTS). In this prior art, APTS is employedto form a silanized surface on SiO₂, which is then used to selectivelyplace the carbon nanotubes.

As known to those skilled in the art, SiO₂ and other oxides ofnon-metals are acidic oxides which form acids when combined with water.Such oxides are known to have low isoelectric points. The term“isoelectric point” is used throughout the present application to denotethe pH at which the net charge on the oxide molecule is zero.

A drawback with the prior art process disclosed in the E. Valentin, etal. article is that the trialkoxysilane undergoes polymerization insolution and self-assembly must be carried out under controlledconditions excluding water. Additionally, APTS cannot be printed usingconventional poly(dimethylsiloxane) (PDMS) stamps in contact printingbecause the solvents that are used for APTS could swell and destroy suchstamps.

In view of the above, there is a continuing need for a method in whichcarbon nanotubes can be selectively placed on substrate surfaces, whileavoiding the drawbacks of the above-described prior art placementprocess, in which APTS is employed.

SUMMARY

The present invention provides a method in which carbon nanotubes can beselectively placed on a predetermined substrate surface, while avoidingthe problems associated with the prior art APTS-based placement process.In particular, the present invention provides a method in whichfunctionalized carbon nanotubes, which are capable of selectivelybonding to metal oxides, are used for selectively placing carbonnanotubes onto a predetermined substrate surface that comprises metaloxide(s). The method of the present invention does not include theformation of a silanized surface for placing the carbon nanotubes, as isthe case in the prior art process described above. Instead, bifunctionalorganic compounds are employed for forming functionalized carbonnanotubes, followed by selective placement of the functionalized carbonnanotubes on metal oxide surfaces.

One aspect of the present invention relates to a method of selectiveplacement of carbon nanotubes on a substrate surface, comprising:

contacting carbon nanotubes with an organic compound that comprises atleast first and second functional groups, wherein the first functionalgroup is capable of forming covalent bonds with carbon nanotubes, andwherein the second functional group is capable of selectively bonding tometal oxides, to form functionalized carbon nanotubes having the organiccompound covalently bound thereto via the first functional group; and

contacting the functionalized carbon nanotubes with a substrate surface,wherein at least one portion of the substrate surface comprises a metaloxide, and wherein the second functional group of the organic compoundselectively bonds to the metal oxide, thereby selectively placing thefunctionalized carbon nanotubes on the at least one portion of thesubstrate surface comprising the metal oxide.

Preferably, the first functional group of the organic compound comprisesat least one aromatic or heteroaromatic functional moiety having fromabout 1 to about 12 rings and bearing at least one diazonium (—N₂ ⁺)salt substituent. More preferably, the first functional group of theorganic compound contains a phenyldiazonium salt functional moiety.

The second functional group preferably comprises at least one organicacid functional moiety, which can selectively bond to metal oxides. Morepreferably, the organic acid functional moiety is selected from thegroup consisting of carboxylic acids, hydroxamic acids, and phosphonicacids, and most preferably, the organic acid functional moiety is —COOH,—C(O)NHOH, or —PO(OH)₂.

The first and second functional groups of the organic compound can belinked together in any suitable manner. For example, such functionalgroups can be linked together via a single covalent bond. In anotherexample, such functional groups can be linked together by a linkerhaving from about 0 to about 20 carbon atoms. Such a linker can have anysuitable configuration, e.g., linear, branched, or cyclic. Preferably,but not necessarily, the first and second functional groups are linkedtogether by a linker selected from the group consisting of —O—, —S—,—NH—, C₁-C₂₀ alkyl, halogenated or partially halogenated C₁-C₂₀ alkyl,C₁-C₂₀ alkyloxy, C₁-C₂₀ alkylthiol, C₁-C₂₀ alkylamino, C₁-C₂₀cycloalkyl, C₁-C₂₀ cycloalkyloxy, C₁-C₂₀ alkenyl, halogenated orpartially halogenated C₁-C₂₀ alkenyl, C₁-C₂₀ alkenyloxy, C₁-C₂₀alkenylthiol, C₁-C₂₀ alkenylamino, C₁-C₂₀ cycloalkenyl, C₁-C₂₀cycloalkenyloxy, C₁-C₂₀ alkyl, and C₁-C₂₀ alkyloxy.

In several particularly preferred embodiments of the present invention,the organic compounds used for forming the functionalized carbonnanotubes are selected from the group consisting of:

wherein n ranges from about 0 to about 20, and wherein X either is asingle bond or is selected from the group consisting of O, S, and NH.

The substrate surface, as used in the present invention, preferablycomprises at least one portion that is coated with a metal oxide layercontaining aluminum oxide and/or hafnium oxide, said metal oxide layerhaving a thickness ranging from about 1 nm to about 100 nm.Specifically, the substrate surface may comprise either an unpatternedmetal oxide layer, or a patterned metal oxide region that is locatedadjacent to or on top of a SiO₂ region.

The functionalized carbon nanotubes of the present invention arepreferably dispersed in a solvent system that comprises one or moreaqueous or organic solvents to first form a dispersion, which is thencontacted with the substrate surface in a suitable manner to allowbonding between the functionalized carbon nanotubes and the metal oxideon the substrate surface.

Subsequently, excess functionalized carbon nanotubes, which have notbonded to the metal oxide, can be removed from the substrate surface byany suitable means. For example, the substrate surface can either bewashed with one or more clean solvents, or be sonicated in one or moreclean solvents. The term “clean” as used herein refers to solvent orsolvents that is/are essentially free of functionalized carbonnanotubes.

Since the functionalized carbon nanotubes have electrical and physicalproperties that are significantly different from pristine carbonnanotubes, it is preferred that additional processing steps are carriedout to “defunctionalize” the selectively placed carbon nanotubes and torestore their superior electrical and physical properties, before suchcarbon nanotubes are used for forming nano-sized electronic devices.

Specifically, the substrate surface is annealed at an elevatedtemperature for an appropriate period of time, so as to remove theorganic compound from the functionalized carbon nanotubes and to formpristine (i.e., clean) carbon nanotubes. The pristine carbon nanotubesso formed are selectively placed on the at least one portion of thesubstrate surface containing the metal oxide, with little or no organiccontamination. The annealing can be conducted, for example, in anitrogen-containing environment, and the annealing temperature may rangefrom about 450° C. to about 650° C. More preferably, the annealingtemperature ranges from about 500° C. to about 600° C. The annealing canbe carried out for from about 60 seconds to about 120 minutes, and morepreferably from about 120 seconds to about 60 minutes.

After formation of the pristine carbon nanotubes, source and draincontacts can be readily deposited over the substrate surface by the wellknown methods, to form field effect transistors that comprises thecarbon nanotubes as channels. In a specific embodiment of the presentinvention, the source and drain contacts are deposited by lithographictechniques.

In another aspect, the present invention relates to a composition thatcomprises one or more functionalized carbon nanotubes. Eachfunctionalized carbon nanotube of the present invention has an organiccompound covalently bound thereto, wherein the organic compoundcomprises at least first and second functional groups, wherein the firstfunctional group is capable of forming covalent bonds with carbonnanotubes, and wherein the second functional group is capable ofselectively bonding metal oxides.

Preferably, but not necessarily, the functionalized carbon nanotubeshave the following formula:

wherein A is an organic acid functional moiety selected from the groupconsisting of —COOH, —C(O)NHOH, and —PO(OH)₂, wherein n ranges fromabout 0 to about 20, wherein y≧1, wherein X either is a single bond oris selected from the group consisting of O, S, and NH, and wherein CNTis a carbon nanotube.

In a further aspect, the present invention relates to an organiccompound comprising at least first and second functional groups, whereinthe first functional group is capable of forming covalent bonds withcarbon nanotubes, and wherein the second functional group is capable ofselectively bonding metal oxides.

In a still further aspect, the present invention relates to an organicprecursor compound having a formula selected from the group consistingof:

wherein n ranges from about 0 to about 20, and wherein X either is asingle bond or is selected from the group consisting of O, S, and NH.

Such an organic precursor compound can be used to form the organiccompound as described hereinabove, which can, in turn, be used to formthe functionalized carbon nanotubes.

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an exemplary prior art back gatefield effect transistor (FET).

FIG. 2 shows a functionalized carbon nanotube with11-phenoxy-1-undecylhydroxamic acid covalent bonded thereto, accordingto one embodiment of the present invention.

FIG. 3 shows selective bonding of the functionalized carbon nanotube ofFIG. 2 to a substrate surface that contains metal oxide.

FIG. 4(A) is a scanning electron microscopic (SEM) photograph that showsfunctionalized carbon nanotubes that are selectively bonded to apatterned Al₂O₃ region on a substrate surface.

FIG. 4(B) is a SEM photograph showing a functionalized carbon nanotubethat is not only selectively bonded to, but also aligned with, a thinstrip of Al₂O₃ on a substrate surface.

FIG. 5(A) is a transmission electron microscopic (TEM) photograph thatshows intact functionalized carbon nanotubes.

FIG. 5(B) is a TEM photograph that shows functionalized carbon nanotubesafter annealing.

FIG. 6 shows the absorption spectra of a functionalized carbon nanotubebefore and after annealing.

FIGS. 7(A)-7(E) shows exemplary processing steps for forming an FET witha carbon nanotube channel by using a functionalized carbon nanotube,according to one embodiment of the present invention.

FIG. 8 shows a SEM photograph of FETs containing carbon nanotubechannels and Pd leads.

FIG. 9 illustrates a synthetic scheme for preparation of a bifunctionalorganic compound containing a hydroxamic acid moiety and a diazoniummoiety.

FIG. 10 shows a synthetic scheme for preparation of a bifunctionalorganic compound containing a phosphonic acid moiety and a diazoniummoiety.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the invention may be practicedwithout these specific details. In other instances, well-knownstructures or processing steps have not been described in detail inorder to avoid obscuring the invention.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present invention provides a selective placement method for carbonnanotubes (CNTs) based on their functionalization, rather than themodification of the substrate surface.

The term “carbon nanotube” is used throughout the present application toinclude a one-dimensional nanomaterial that has a hollow cavity withnanometer-sized diameters and much, much longer lengths. In other words,the carbon nanotubes have a high aspect ratio and quantum effects becomeimportant for these systems. The nanotubes that can be used in thepresent invention are single walled or multi-walled nanomaterials thattypically have an outer diameter that is typically from about 0.8 nm toabout 30 nm, with an outer diameter from about 1.0 nm to about 2.5 nmbeing more typical, and a length that is typically from about 5 nm toabout 100 μm, with a length from about 10 nm to about 10 μm being moretypical. In addition to having an outer diameter, the nanotubes that canbe used in the present invention have an inner diameter that istypically from about 0.8 nm to about 15 nm, with an inner diameter fromabout 0.8 nm to about 2.5 nm being more highly typical. The nanotubesuseful in the present invention are further characterized as having ahigh aspect ratio that is typically on the order of about 5 or greater,with an aspect ratio from about 5 to about 5000 being typical.

The nanotubes formed include a C-based nanomaterial that has a hexagonallattice structure that is rolled up.

The carbon nanotubes used in the present invention are made usingtechniques well known to those skilled in the art. For example, thecarbon nanotubes can be formed by laser ablation, chemical vapordeposition (CVD) of various organic materials, such as carbon monoxide,methane, and ethanol, and electrical discharge.

One embodiment of the present invention involves forming functionalizedcarbon nanotubes by using a group of novel bifunctional organiccompounds that have been designed and synthesized by the inventors ofthe present invention. Specifically, each of such organic compoundscontains at least two functional groups, the first of which can formcovalent bond with carbon nanotubes, and the second of which can bondselectively to metal oxides, but not to silicon oxide. Such bifunctionalorganic compounds, after being covalently bonded to carbon nanotubes viatheir first functional groups, effectively functionalize such carbonnanotubes with their metal-oxide-bonding second functional groups.Therefore, the carbon nanotubes so functionalized can selectively bondto substrate surfaces that contain metal oxides (via the secondfunctional groups of the organic compounds), but not to substratesurfaces that contain silicon oxide. Excess carbon nanotubes that havenot formed selective bonds with the metal oxides can then be removedfrom the substrate surface.

The first functional group of the organic compounds of the presentinvention can comprise an aromatic or heteroaromatic functional moietyhaving at least one diazonium salt substituent, with or withoutadditional substituent(s). The aromatic or heteroaromatic ring of thisfirst functional group can form a carbon-carbon single bond with acarbon nanotube with the assistance of the at least one diazonium saltsubstituent, thereby covalently bonding such an organic compound to thecarbon nanotube.

Preferably, the aromatic or heteroaromatic functional moiety has fromabout 1 to about 12 rings. The heteroaromatic functional moiety caninclude one of the following as the heteroatom: nitrogen, sulfur, oxygenor combinations thereof. The aromatic or heteroaromatic functionalmoiety contains a diazonium salt substituent (—N₂ ⁺), which can reactwith a carbon nanotube to facilitate the formation of the carbon-carbonsingle bond between the carbon nanotube and the aromatic orheteroaromatic ring.

More preferably, the first functional group may comprise anaryldiazonium functional moiety (—Ar—N₂ ⁺) or a substitutedaryldiazonium functional moiety with one or more additionalsubstituents. Most preferably, the first functional group comprises aphenyldiazonium moiety or a substituted phenyldiazonium moiety with atleast one alkyl substituent, as follows:

wherein R is an alkyl group with from about 1 to about 12 carbon atoms.

The aryldiazonium functional moiety, as mentioned hereinabove, can bereadily formed using a precursor organic compound that contains anaminoaryl functional moiety (—Ar—NH₂), which can react with nitrosoniumtetrafluoroborate (NO⁺BF₄ ⁻) in acetonitrile to form a correspondingaryldiazonium tetrafluoroborate (—Ar—N₂ ⁺BF₄ ⁻) functionality.

The second functional group of the organic compounds of the presentinvention can comprise any suitable organic acid functional moiety thatcan bond to metal oxides in selection to silicon oxide. Preferably, butnot necessarily, the second functional group comprises a functionalmoiety selected from the group consisting of carboxylic acids,hydroxamic acids, and phosphonic acids.

The first and second functional groups of the organic compounds caneither be directly linked together by a single covalent bond, or belinked together by a linker, which can comprise from 0 to 20 carbonatoms and which can be linear, branched, or cyclic. Preferably, thefirst and second functional groups of the organic compounds are linkedtogether by a linker selected from the group consisting of —O—, —S—,—NH—, C₁-C₂₀ alkyl, halogenated or partially halogenated C₁-C₂₀ alkyl,C₁-C₂₀ alkyloxy, C₁-C₂₀ alkylthiol, C₁-C₂₀ alkylamino, C₁-C₂₀cycloalkyl, C₁-C₂₀ cycloalkyloxy, C₁-C₂₀ alkenyl, halogenated orpartially halogenated C₁-C₂₀ alkenyl, C₁-C₂₀ alkenyloxy, C₁-C₂₀alkenylthiol, C₁-C₂₀ alkenylamino, C₁-C₂₀ cycloalkenyl, C₁-C₂₀cycloalkenyloxy, C₁-C₂₀ alkyl, and C₁-C₂₀ alkyloxy. More preferably, thelinker is a linear C₁-C₂₀ alkyloxy, and most preferably, the linker iseither an undecyloxy or a dodecyloxy.

Particularly preferred organic compounds of the present invention are:

wherein n ranges from about 0 to about 20, and wherein X either is asingle bond or is selected from the group consisting of O, S, and NH.

These preferred organic compounds can be readily formed from thefollowing precursor compounds:

by reacting the precursor compounds with nitrosonium tetrafluoroborate(NO⁺BF₄ ⁻) in acetonitrile.

The organic compounds as described hereinabove are then contacted withcarbon nanotubes to form functionalized carbon nanotubes with theorganic compounds covalently bonded thereto. FIG. 2 illustratively showsa functionalized carbon nanotube with two 11-phenoxy-1-undecylhydroxamicacid molecules covalent bonded thereto.

In a particularly preferred embodiment of the present invention, thefunctionalized carbon nanotubes are dispersed in one or more aqueous ororganic solvents, to form a dispersion of carbon nanotubes. Thedispersion can be readily prepared using techniques that are well knownin the art. Typically, the dispersion is prepared by sonication ofcarbon nanotubes in an organic solvent (such as, for example, C₁-C₃alcohols, dichloroethylene, N-methylpyrolidone or dichloromethane) or inan aqueous solution that contains from about 0.1 to about 1% of asurfactant. Examples of surfactants that can be used in preparing theaqueous dispersion of carbon nanotubes include sodiumdodecylbenzenesulfonic acid (SDS) and poly(oxyethylene)-substitutedaromatic compounds such as Triton N-100 or Triton X-100.

The functionalized carbon nanotubes can then be contacted with asubstrate surface that comprises at least one portion containing metaloxides. When a dispersion of the functionalized carbon nanotubes hasbeen provided, the substrate surface can be simply immersed in such adispersion for a sufficient period of time to allow the secondfunctional groups of the covalently bonded organic compounds toselectively bond to the metal oxides. In this manner, the functionalizednanotubes are selectively placed on the metal-oxide-containing portionof the substrate surface. FIG. 3 illustratively shows selective bondingof the functionalized carbon nanotube of FIG. 2 to a substrate surfacethat contains metal oxide.

The metal oxide of the present invention includes at least one metalfrom group IVB, VB, VIB, VIIB, VIII or IIA (CAS version) of the PeriodicTable of Elements. More preferably, the metal oxide of the presentinvention is selected from Al₂O₃, HfO₂, TiO₂, SnO₂ or ZrO₂. The metaloxide 10 may be located atop another dielectric material or asemiconducting material.

The substrate surface may comprise either a uniform, unpatterned metaloxide layer, or a patterned metal oxide region located adjacent to or ontop of a SiO₂ region. Preferably, the substrate surface comprises ametal oxide layer over at least one portion thereof, while the metaloxide layer comprises aluminum oxide and/or hafnium oxide and has athickness ranging from about 1 nm to about 100 nm.

After the selective placement of the functionalized carbon nanotube,excess functionalized carbon nanotubes that have not yet bonded to themetal oxides are removed from the substrate surface. The removal can becarried out by any suitable methods. For example, the substrate surfacecan be washed with one or more clean solvents, or it can be sonicated inone or more clean solvents.

The selective placement method as proposed by the present invention notonly achieves excellent site-specific bonding of the functionalizedcarbon nanotubes to the metal-oxide-containing surface regions, but alsocan be used to align the functionalized carbon nanotubes to the contoursof narrow metal-oxide-containing surface features. FIG. 4(A) shows a SEMphotograph of functionalized nanotubes (the thin white threads) thathave been selectively deposited onto a SiO₂ substrate with a patternedAl₂O₃ surface region. The patterned Al₂O₃ region has a dense nanotubelayer, but other regions of the silicon oxide substrate do not containany adsorbed nanotubes. FIG. 4(B) shows a SEM photograph of a SiO₂substrate with a narrow Al₂O₃ strip thereon, while a functionalizednanotube (pointed to by the white arrowheads) has been selectivelyplaced on the narrow Al₂O₃ strip in a substantially aligned manner.

Following selective placement of the functionalized carbon nanotubes,additional processing steps can then be performed to remove thecovalently bonded organic compound, thereby “defunctionalize” the carbonnanotubes and restore their superior physical and electrical properties.

One important advantage of the present invention is that thefunctionalized carbon nanotubes can be converted into pristine carbonnanotubes by annealing, with little or no impact on their physical andelectrical properties.

FIG. 5(A) shows the TEM photographs of carbon nanotubes that wereuniformly and completely functionalized, which were dispersed inmethanol on “holey” SiO₂ coated TEM grids. A dense, amorphous layer oforganic compounds covered each carbon nanotubes.

However, after annealing in a nitrogen-containing environment at anannealing temperature from about 500° C. to about 600° C. for about 120seconds to about 60 minutes, the amorphous layer disappeared, and thecarbon nanotubes appeared remarkably clean, with little or no defects.The structural integrity of the nanotubes is maintained during theannealing process.

Atomic force microscopic (AFM) measurements (not shown here) furtherconfirms that the annealing resulted in a complete removal of theorganic compounds, with little or no residual contamination on thenanotube surfaces. Before the annealing, the average tube diameter wasabout 1.8 nm, and it reduced to about 1.0 nm after the annealing.

Because the one-dimensional character of carbon nanotubes leads to theformation of strongly bound excited states. The lowest allowed excitonoccurs in the infra-red region, which can be easily detected by usingadsorption spectroscopy. FIG. 6 shows the absorption spectra of afunctionalized carbon nanotube before and after annealing. The spectrumrecorded for the functionalized carbon nanotube before annealing wasfeatureless, indicating that the band structure of the functionalizednanotube is very different from that of a pristine (or clean) nanotube.However, after annealing, a broad adsorption band around 1650 nmappeared, associated with the first dipole active exciton. The broadshape of the absorption peak is due to the presence of a distribution ofnanotubes in the sample, with diameters ranging from about 0.8 nm toabout 1.2 nm. The appearance of this band upon annealing indicates thatthe one-dimensional band structure of the nanotube is restored, mostlikely because the covalent bonds between the nanotubes and the organiccompounds were broken by the annealing. The spectra obtained for thefunctionalized carbon nanotubes after annealing are similar to thoseobtained for pristine nanotube samples.

The specific annealing conditions for practicing the present inventioncan be varied widely, depending on the specific types of carbonnanotubes used. For nanotubes with an average tube diameter ranging fromabout 0.8 nm to about 1.2 nm (measured before the functionalization),the annealing temperature may range from about 450° C. to about 650° C.,more preferably from about 500° C. to about 600° C., and the annealingduration may range from about 60 seconds to about 120 minutes, and morepreferably from about 120 seconds to about 60 minutes.

The selectively placed carbon nanotubes can then be used for fabricatingnano-sized FETs or other electronic devices. For example, FET source anddrain contacts can be deposited by lithography over the substratesurface in direct contact with the carbon nanotubes, so as to form FETswith carbon nanotube channels.

FIGS. 7(A)-7(E) shows exemplary processing steps for forming an FET witha carbon nanotube channel by using a functionalized carbon nanotube,according to one embodiment of the present invention.

Specifically, a SiO₂ substrate 22 is provided, as shown in FIG. 7(A),which is then patterned with a narrow Al strip (not shown) of about 40nm thick and 300 nm wide. The substrate 22 is then exposed to an oxygenplasma (e.g., for 3 minutes at 600 mTorr), so as to oxidize the surfaceof the Al strip, to form an Al₂O₃ surface layer 24, as shown in FIG.7(B). The Al₂O₃ surface layer 24 protects the underlying Al materialfrom being oxidized and also separates the underlying metal from thecarbon nanotube to be deposited. A functionalized carbon nanotube 26 inaccordance with the present invention is then selectively bonded to theAl₂O₃ surface layer 24, as shown in FIG. 7(C). Subsequent annealing inN₂ removes the organic compound from the functionalized carbon nanotube26, thereby forming a pristine carbon nanotube 26′ on the Al₂O₃ surfacelayer 24, as shown in FIG. 7(D). Finally, a lithographic step is carriedout to deposit source and drain contacts 28(a) and 28(b) over thesubstrate 24 in direct contact with two ends of the carbon nanotube 26′.In this manner, a nano-sized FET device is formed, in which theunreacted Al metal functions as the back gate, the Al₂O₃ surface layer24 functions as the gate dielectric, and the carbon nanotube 26′functions as the channel.

FIG. 8 further shows a SEM photograph of actual FETs formed by a processsimilar to that described hereinabove. The vertical leads are Al/Al₂O₃gate structures with nanotubes adsorbed thereon, and the horizontalleads are Pd contacts that are in direct contact with the carbonnanotubes.

The channel length of the FETs formed by the method of the presentinvention typically ranges from about 50 nm to about 1000 nm, moretypically from about 100 nm to about 500 nm, and most typically fromabout 350 nm to about 450 nm.

The following examples are provided to illustrate the various processingschemes of the present invention for the selective placement of carbonnanotubes.

EXAMPLE 1 Preparation of a Bifunctional Organic Compound with aHydroxamic Acid Functional Moiety and a Diazonium Functional Moiety

FIG. 9 shows the synthetic scheme for preparation of a bifunctionalcompound that comprises a hydroxamic acid moiety and a diazonium moiety.First, 11-bromo-1-undecanoic acid (9 a) is converted to correspondingacid chloride (9 b), which is then reacted with O-benzylhydroxylamine toform O-benzyl-10-bromodecylhydroxamic acid (9 c). Reaction of 9 c with4-nitrophenol in the presence of potassium carbonate results in theformation of O-benzyl-10-(4-nitrophenoxy)decylhydroxamic acid (9 d).Hydrogenation of 9 d forms 10-(4-aminophenoxy)decylhydroxamic acid (9e), which reacts with nitrosonium tetrafluoroborate in acetonitrile toform the compound 9 f, which contains a diazonium salt moiety at one endand a hydroxamic acid moiety at the other end.

In a specific experiment, oxallyl chloride (0.02 mole) was added to asolution of 0.01 mole 10-bromo-1-decanoic acid (9 a) in dichloromethanecontaining traces of N,N-dimethylformamide and stirred for 4 hours.Excess oxallyl chloride was removed under reduced pressure, and theremaining oily acid chloride was dissolved in 50 mL dichloromethane andadded to a solution of O-benzylhydroxylamine in dichloromethanecontaining 0.01 mole of triethylamine and the mixture was stirred atroom temperature. The mixture was washed with dilute hydrochloric acidand brine, dried over anhydrous magnesium sulfate and the solvent wasremoved under reduced pressure to give desired protected hydroxamic acid(i.e., O-benzyl-10-bromodecylhydroxamic acid of 9 c). Crystallizationfrom ethanol afforded pure sample of 9 c.

Potassium carbonate (5.0 grams) was added to a solution of 4-nitrophenol(0.01 mole) and the compound 9 c (0.01 mole) in N,N-dimethylformamide(10 mL) and the mixture was heated at 110° C. under nitrogen for 18hours. The mixture was cooled to room temperature. Water (100 mL) wasadded and extracted with diethyl ether. The ether extract was washedwith dilute potassium hydroxide solution, brine and dried over anhydrousmagnesium sulfate and evaporated under reduce pressure. The solidresidue was crystallized from toluene to give the compound 9 d (i.e.,O-benzyl-10-(4-nitrophenoxy)decylhydroxamic acid) as light yellowcrystals.

Palladium on carbon (10%, 200 mg) was added under nitrogen to a solutionof the compound 9 d (0.01 mole) and ammonium formate (0.05 mole) inanhydrous methanol (50 mL), and the mixture was heated to reflux for 4hours. The solution was filtered, and the solvent was removed to giveform the compound 9 e (10-aminophenoxy-1-decylhydroxamic acid) as whitecrystalline compound.

EXAMPLE 2 Preparation of a Bifunctional Organic Compound with aPhosphonic Acid Functional Moiety and a Diazonium Functional Moiety

FIG. 10 shows the synthetic scheme for preparation of a bifunctionalcompound that comprises a phosphonic acid moiety and a diazonium moiety.First, 11-Bromo-1-undecanol (10 a) is esterified with acetyl chlorideand then reacted with triethylphosphite to form the phosphonate (10 c).Deprotection of acetyl group and reaction of 4-nitrophenol in thepresence of triphenyl phosphine and diethyl azodicarboxylate results inthe formation of 11-nitrophenoxyundecyl phosphonate (10 d). Reduction ofnitro group to amine and hydrolysis of phosphonate to phosphonic acidthen form O-benzyl 11-(4-nitrophenoxy)undecylphosphonic acid (10 e).Hydrogenation of 10 e can then form 11-(4-aminophenoxy)undecylhydroxamicacid (not shown), which in turn reacts with nitrosoniumtetrafluoroborate in acetonitrile to form a compound (not shown) thatcontains a diazonium salt moiety at one end and a phosphonic acid moietyat the other end.

EXAMPLE 3 Preparation of a Dispersion of Functionalized Carbon Nanotubes

A solution of the compound 9 e (60 mg) in anhydrous acetonitrile wasadded to a cold solution of nitrosonium tetrafluoroborate (18 mg) inanhydrous acetonitrile, and the mixture was stirred at room temperature.The yellow solution was added to a dispersion of carbon nanotubes in 1%aqueous sodium dodecylsulfonate and stirred at room temperature for 18hours. The diazonium salt of 10-(4-aminophenoxy)-1-decylhydroxamic acidfunctionalized the carbon nanotubes by forming a covalent carbon-carbonbond with the carbon nanotube through the phenyl ring, withoutdestroying the lattice structure of the carbon nanotubes.

The functionalized nanotubes were then isolated by addition of largeamounts of acetone, followed by centrifugation. Supernatant liquid wasdiscarded, and the precipitated carbon nanotubes were then dispersed inmethanol to form stable dispersion of functionalized carbon nanotubes,which stayed isolated in the dispersion for several weeks.

EXAMPLE 4 Selective Bonding of Functionalized Carbon Nanotubes toSubstrate Surface with Metal Oxide

A SiO₂ substrate patterned with Al₂O₃ was first formed, by firstdepositing a patterned Al film over the substrate, followed by oxidizingthe Al film using an oxygen plasma. The patterned Al₂O₃ layer was about4 nm thick. The substrate was then immersed in a dispersion offunctionalized carbon nanotubes in methanol, as described in EXAMPLE 3,and heated to 50° C. The substrate was subsequently removed from thedispersion and sonicated in clean methanol, followed by drying in astream of nitrogen. A deposition of functionalized carbon nanotubesexclusively on aluminum oxide surface was resulted.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A composition comprising a dispersion of one or more functionalizedcarbon nanotubes having an organic compound covalently bound thereto ina solvent, wherein said organic compound comprises at least first andsecond functional groups, wherein the first functional group is capableof forming covalent bonds with carbon nanotubes, and wherein the secondfunctional group is capable of selectively bonding metal oxides, whereinthe functionalized carbon nanotubes have the following formula:

wherein A is an organic acid functional moiety selected from the groupconsisting of —C(O)NHOH, and —PO(OH)₂, wherein n ranges from about 0 toabout 20, wherein y≧1, wherein X either is a single covalent bond or isselected from the group consisting of O, S, and NH, and wherein CNT is acarbon nanotube.
 2. The composition of claim 1, wherein A is —C(O)NHOH.3. The composition of claim 1, wherein A is —PO(OH)₂.
 4. The compositionof claim 1, wherein said solvent is aqueous.
 5. The composition of claim4, further comprising 0.1 to 1% of a surfactant.
 6. The composition ofclaim 1, wherein said solvent is an organic solvent selected from thegroup consisting of C₁-C₃ alcohols, dichloroethylene, N-methylpyrolidoneand diclhorosilane.
 7. The composition of claim 1, wherein X is O. 8.The composition of claim 1, wherein X is S.
 9. The composition of claim1, wherein X is NH.
 10. The composition of claim 1, wherein A is—C(O)NHOH, X is O and n equal
 11. 11. The composition of claim 1,wherein A is —PO(OH)₂, X is O and n equal
 11. 12. A compositioncomprising one or more functionalized carbon nanotubes, wherein thefunctionalized carbon nanotubes have the following formula:

wherein A is an organic acid functional moiety selected from the groupconsisting of —C(O)NHOH, and —PO(OH)₂, wherein n ranges from about 0 toabout 20, wherein y≧1, wherein X either is a single covalent bond or isselected from the group consisting of O, S, and NH, and wherein CNT is acarbon nanotube.
 13. The composition of claim 12, wherein A is—C(O)NHOH, X is O and n equal
 11. 14. The composition of claim 12,wherein A is —PO(OH)₂, X is O and n equal
 11. 15. The composition ofclaim 12, wherein said functionalized carbon nanotubes are present as adispersion within a solvent.
 16. The composition of claim 12, whereinsaid solvent is aqueous.
 17. The composition of claim 16, furthercomprising 0.1 to 1% of a surfactant.
 18. The composition of claim 15,wherein said solvent is an organic solvent selected from the groupconsisting of C₁-C₃ alcohols, dichloroethylene, N-methylpyrolidone anddiclhorosilane.
 19. A composition comprising one or more functionalizedcarbon nanotubes, wherein the functionalized carbon nanotubes have thefollowing formula:

wherein A is —C(O)NHOH, wherein n ranges from about 0 to about 20,wherein y≧1, wherein X either is a single covalent bond or is selectedfrom the group consisting of O, S, and NH, and wherein CNT is a carbonnanotube.